Level control using alternating current through material of variable resistance



USING ALTERNATING CURRENT N. HARTLEY Filed May 2, 1957 LEVEL CONTROLTHROUGH MATERIAL OF VARIABLE RESISTANCE April 5, 1960 v INVENTOR. N62 50flier-4.5 v

ATTOKA/E Y5 VVV I as LEVEL CONTROL USING ALTERNATING CUR- RENT THROUGHMATERIAL OF VARIABLE RESISTANCE Nelson Hartley, Baltimore, Md., assignorto Hartley Controls Corporation, Neenah, Wis., a corporation ofWisconsin Application May 2, 1957, Serial No. 656,593

9 Claims. (Cl. 214-17) This invention relates to level control usingalternating current through material of variable resistance. Control ofsand level in amolders hopper is disclosed by way of exemplification.

The arrangement is such that sand is supplied to the hopper when thelevel drops below one probe and the supply is cut off when the sandreaches the level of an upper probe.- Current fromthe respective probespasses through the sand to another electrode which here comprises thehopper wall.

It has been supposed that alternating current would not be practical insuch an installation because the resistance of the sand varies widelyaccording to its composition, physical structure, temperature, and watercontent. Sand having low conductivity would normally'have to build up tocover a greater area of the upper probe before there would be sufficientflow of current to actuate the control relay. Meantime, as the currentin the relay approached the desired value, the alternating current wouldcause the armature to chatter and the contacts would become pitted dueto the excessive arcing consequent upon operation in this manner.

According to the present invention the primary relay does not have to bespecially designed or specially sensitive. It is an ordinary double polerelay. Its coil is connected to be energized from the high probe in thehopper and one of its contacts sets up a holding circuit to the coilthrough the other probe so that the relay contacts do not open until thesand clears the lower end of the longer probe means. The use ofalternating current is made practical by providing a shunt circuit tothe relay coil by-passing the probes and the sand and including avariable resistance. This can be set to supply nearly all currentrequired to operate the relay, so that the remaining requirement is sosmall that the relay will close immediately regardless of sandresistance and yet will open immediately when the sand clears the lowerprobe.

The alternating current is very valuable in this device because iteliminates electrolytic attack on the electrodes (the probes or hopper)as experienced in the use of'D.C. probe circuits.

As specifically applied to molding sand practice, the electrical systemmay control a plow which is movable to and from a supply belt and which,in its operative position, deflects sand from the belt into the moldershopper. In the preferred embodiment, a time delay feature isincorporated in-the circuit so that even after the sand level reachesthe upper probe, the plow will continue effective until all sandremaining on the supply belt is sloughed olf, This feature is used inconjunction with means for interrupting at the source the movement ofsand onto the supply belt, the object being to leave the supply beltfree of sand between hopper filling operations, thereby preventing sandfrom drying out due to exposure on the belt. 1 1 a In the drawings:

Fig. l is a diagrammatic view of the apparatus and circuits employed. r

nited States Patent Fig. 2 is a fragmentary diagrammatic view showing amodified embodiment.

The prepared sand is stored in the supply hopper 5 and issues therefromonly when the conveyor belt 6 is in operation across the mouth of thehopper to discharge sand onto the conveyor belt 7 which delivers it toone or more individual molders hoppers 8, 9. Belt 7 may be assumed to bein continuous operation by means of motor 10. The control belt 6 isactuated by motor 11 which is started and stopped in accordance with thepreferred embodiment of the present invention.

In accordance with conventional practice, a deflector or plow 12 ismovable between retracted and operative positions respecting belt 7 ateach of the molders hoppers 8, 9. The plow is commonly operatedpneumatically by means of air from a supply pipe 13 admitted to one endor the other of the cylinder 14 to raise or lower the ram plunger 15upon which the plow is mounted. The air supply is controlled by asolenoid valve 16.

In accordance with the present invention short and long probes 2t) and21 are placed in the respective hoppers 8, 9, which serve as containersfor the material to be measured. For illustrative purposes, the materialcornprises the sand shown at 22 in the supply hopper 5 and on the belt 7and in the respective hoppers. The short probe 20 represents a means ofdetermining the upper level to which the material 22 is to be filledinto the container. The longer probe 21 is a means of determining whenthe re-filling operation is to be initiated.

One side of the alternating current supply source 25 is connected to aground lead 26. The hot side of the supply source is connected throughthe contactor 27 of the double pole main switch 28 to the line 29 whichenergizes the transformer 30 and also supplies current, subject tocontrol of normally closed relay contactor 31, to motor 11 whichactuates the conveyor 6 for the control of sand delivery from the supplyhopper 5.

The secondary of transformer 30 is connected through conductor 33 to thecontainer 8 which is used as an electrode exposed to the sand 22. Theother side of the secondary is connected through conductor 34 and relaycoil 35 and conductor 36 to the upper probe 20. When a sufficient lengthof probe 20 is exposed to the material 22 in container 8, there will bea flow of current through the material tending to attract the armature38 of relay coil 35 to move normally open contactors 40 and 41 to closedcircuit position, this being the status of the armature and contactorsas illustrated in the drawing.

The primary relay armature 38 would chatter and its contacts would bepitted and short lived if the alternating current through the sand weresolely relied upon for energizing coil 35. The resistance of the sand ishighly variable and use of alternating current would not be practicableif the flow through the coil had to build up slowly during the fillingof the hopper to a value sutficient to actuate the armature under allconditions. Accordingly, ad-

justable resistor 37 is set to permit flow through shunt circuit 39 ofalmost enough current to close the contacts 40 and 41 but not enough tohold them closed.

With this shunt, there is substantially immediate response of armature38 in a circuit closing direction when the sand touches the probe 20,without the chattering and pitting which would be the consequence if thecurrent had to be built up to an effective value as the sand graduallyclimbs up probe 20. On the other hand, the armature immediately dropsopen when the sand clears probe 21. Thus there is no indecisive actioneither in the energization or de-energization of this relay and thismakes it practicable to use alternating current with just an ordinaryconventional relay. No unusual relay sensitivity or relay adjustment isrequired because the shunt circuit is adjustable to provide the desiredoperation.

For example, and not by way of limitation, the relay may trip when 300milliamperes flows through its coil. Of this current, perhaps 270milliarnperes may be provided by the shunt circuit and only 30, more orless, by the electrode in the sand.

Contactor 40, in its closed position, completes a holding circuit fromthe longer probe 21 through conductor 42 to the relay coil 35. In otherwords, the response of the relay achieved by engagement of the sand withthe upper probe 20 will continue effective as long as the sand remainsin contact with the longer probe 21.

The result of the described operation of the relay has been to close thecircuit controlled by relay contactor 41 between the supply conductor 29and the relay coils 43 and 44. In the illustrated position of the parts,the energization of coil 43 has already resulted in the movement of thecontactor 31 to its open circuit position, thereby interrupting theoperation of motor 11. As appears in the drawing, sand is no longerbeing delivered from the supply hopper to the distributing belt 7. Thecontactor 45 controlled by coil 44 remains closed due to the provisionof a time delay mechanism 46. This contactor controls the flow ofcurrent from conductor '29 through conductor 47 to the solenoid valve16. As long as the time delay relay remains closed, air from the supplyline 13 will continue to be communicated to the cylinder 14 in adirection to maintain the normally retracted plow 12 effective todeflect sand from the distributing belt 7 into the hopper 8 in order toclear the belt.

The upper probe is set at such a level in the molders supply hopper 8 asto allow sand to be delivered to that particular hopper during theperiod for which the time delay relay will remain active. The ordinarytime delay relay is also adjustable and either the time adjustment orthe setting of the upper probe may be used to ensure the provision ofadequate space in the hopper 8 to receive the sand which is deliveredthereto for belt clearing purposes after the flow of sand to the belthas been interrupted.

The molder will withdraw sand from hopper 8 through gate 48 as requiredfor his work. As the sand level drops in hopper 8, there will continueto be a flow of current from probe 21 to the hopper wall or otherelectrode to enable the circuit through coil to maintain the relayarmature 38 in the illustrated circuit closing position. However, whenthe sand level falls below the lower end of the long probe 21, theholding circuit will be broken, armature 38 will move to its normal opencircuit position at the left of that illustrated, thereby causingcontactor 41 to open the circuit controlled by it while contactor opensthe circuit through coil 35.

Thus, every time there is a demand for additional material to re-fillthe container 8, the coils 43 and 44 will be tie-energized to initiate aflow of material toward container 8. In the instant device, this isaccomplished by starting the motor 11 to deliver material from thesupply hopper 5 and to lower the deflecting plow 12 into operativeposition for deflecting the material into the hopper 8 from thecontinuously acting conveyor 7. When the material reaches the depthillustrated, the response of coil 35 to the flow of current from probe20, supplemented by that through the adjustable resistor 37, closes thecircuit to relay 43, 44 to open the circuit to the material supplymeans. The holding circuit through contactor 40 prevents the supplymeans from operating until the material falls below the depth of thelonger probe.

It will be understood that each successive hopper served by thedistributing belt 7 will be provided with a circuit identical with thatwhich controls the delivery of the material to hopper 8. However, itwill be observed that the upper probe in hopper 9 is somewhat deeperthan the upper probe in hopper 8 because, being served by a greaterlength of belt 7, space must be provided for unloading into hopper 9 agreater amount of material from the belt after the tripping of the relay43 than was required to be delivered into hopper 8, which is closer tothe source.

In installations in which only a single control of delivery of materialinto the container may be required, the time delay relay can bedispensed with. This is shown in Fig. 2, wherein the'coil '43 alonesuflices 'to actuate the solenoid 16 for the purposes described. It isdesired again to emphasize the fact that it is immaterial in the broaderaspects of the invention whether the responding device which controlsthe delivery of the material comprises the plow 12 or something else.However, while the invention has broad aspects, it does not minimize itsimportance in the particular field of the illustrated embodiment,wherein it solves problems of long standing and greatly reduces costs ascompared with conventional means of controlling by hand thereplenishment of sand in the molders hoppers.

I claim: g

1. In combination a container for an electrically conductive materialprovided with an electrode connection, electrically controlled means forfilling the container, a high level electrode in the container in aposition to be in contact with material nearly filling the container anda controlling relay for said filling means and including an actuatingcoil having a series connection with the high level electrode, saidrelay including contactor means for energizing the filling means, asource of current in circuit with said electrodes and coil, and aby-pass circuit connection between the source and coil independently ofthe electrodes and material and having an electrical current carrying,characteristic 'so related to the relay-actuating characteristic of thecoil as to supply the major part of the current required for relayoperation.

' 2. The device of claim 1 in which the current source is a source ofalternating current.

3. The device of claim 2 in further combination with an elongatedelectrode in the container extending to a level at which material in thecontainer is to be replenished, said relay including a holding circuitincluding said elongated electrode and said coil and for which saidrelay includes a circuit closing contactor means, whereby the relay willcontinue operative to deenergize the refilling means until the level ofmaterial in the container falls below the longer electrode.

4. The device of claim 2 in which the by-pass circuit connection betweenthe source of thecoil includes adjustable means for varying the flow ofcurrent through the by-pass circuit.

5. The device of claim 2 in which the filling means comprises a plowmovable between operative and retracted positions and provided withelectric means for elfecting such movement and with a supply belt withwhich the plow is in close association in its operative position todeflect material from the belt into the coutainer. I

6. The device of claim 5 in further combination with means subject tothe control of said relay for delivering material to the belt to beconveyed thereby to said plow.

7. The device of claim 6 in further combination with a time delay relayconnected between the first mentioned relay and the electrical means foractuating the plow, said time delay relay being adapted to maintain theplow in its operative position substantially for such a period as may berequired to clear the belt after the material in the container contactsthe high level electrode.

8. The combination with a molders hopper and a sand delivering conveyor,of a plow movable between retracted position and an operative positionrespecting the conveyor for deflecting sand from the conveyor into thehopper, electrically operable means for determining the position of theplow, an elongated probe in the hopper extending to the desired minimumsand level therein, a high level probe within the hopper, the hopperbeing provided with an electrode connection and the respective probesbeing provided with electrical connections, a relay having contactormeans, an armature connected with said means, and a coil, circuitconnections including a supply source in which the coil is connected inseries between the high level probe and the electrode connectionaforesaid, holding circuit connections including said source and inwhich the coil is connected in series subject to the control of saidcontactor means between the elongated probe and the electrode connectionaforesaid, an operating circuit connection including a supply source andsubject to the control of the contactor means of said relay and leadingto the means for electrically controlling the position of the plow, thelast mentioned circuit connections including a holding relay formaintaining the plow in operative association with the conveyor afterthe first mentioned relay has been actuated by current flow through thehigh level probe to a position which would otherwise render the plowimmediately inactive by effecting movement thereof to its retractedposition.

9. The combination with a molders hopper in which it is desired torestore molding sand to a predetermined level after the sand has fallento a predetermined low level, the said hopper having an electrodeconnection, of an electrically conductive high level probe within thehopper, an elongated electrically conductive probe extending within thehopper to the level at which replenishment is desired, electricallycontrolled means'for delivering sand to the hopper, and a control relaycomprising 'a coil, an armature and first and second contactors, an

electrical circuit including the high level probe, the relay coil, asource of alternating current and the electrode connection in series,whereby the relay coil is energized by a flow of current to the sand inthe hopper when the sand contacts the high level probe, circuit meanscontrolled by the first contactor and closed upon the energization ofsaid coil to connect said coil and said elongated probe and currentsource in series to hold said coil energized as long as the sand in thehopper remains in contact with the elongated probe, and circuit meanscontrolled by the second contactor and elfective when the coil isde-energized to connect in series said source of current and the meansfor electrically controlling delivery of sand to the hopper, wherebysaid last mentioned means will become eflective to initiate delivery ofsand as soon as the sand falls below the level of the elongated probeand will remain effective at least until the level of sand in the hopperreaches the level of the high level probe.

References Cited in the file of this patent UNITED STATES PATENTS1,446,961 Brown Feb. 27, 1923 2,542,239 Engels et al. Feb. 20, 19512,614,707 Bilterman Oct. 21, 1952 FOREIGN PATENTS 623,970 Great BritainMay 25, 1949 628,336 Great Britain Aug. 26, 1949

